Heart failure is a debilitating disease in which abnormal function of the heart leads in the direction of inadequate blood flow to fulfill the needs of the tissues and organs of the body. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately eject or fill with blood between heartbeats and the valves regulating blood flow become leaky, allowing regurgitation or back-flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness and the inability to carry out daily tasks may result. Not all heart failure patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive. As heart failure progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles (particularly the left ventricle) to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart's oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output. A particularly severe form of heart failure is congestive heart failure (CHF) wherein the weak pumping of the heart leads to build-up of fluids in the lungs and other organs and tissues.
Pulmonary edema occurs when the alveoli of the lungs fill with fluid instead of air, preventing oxygen from being absorbed into the bloodstream. That is, when a diseased or overworked left ventricle is no longer able to pump enough of the blood it receives from the lungs, pressure thereby increases inside the left atrium and then in the pulmonary veins and capillaries, causing fluid to be pushed through the capillary walls into the air sacs. This can cause severe respiratory problems and, left untreated, can be fatal. Note that pulmonary edema can also arise due to other factors besides heart failure, such as infections. Pulmonary edema due to heart failure may be specifically referred to as cardiogenic pulmonary edema. Herein, for brevity, the term pulmonary edema will be used to refer to cardiogenic pulmonary edema.
In general, whenever the right ventricle pumps more blood than the left ventricle is capable of pumping (for whatever reason), pulmonary congestion will arise. It would be desirable to exploit this insight to provide techniques for detecting pulmonary congestion and edema, and it is to that end that the invention is generally directed.